Hydraulic pumps with an inclined plate, or swash plate, driving axial pistons have been known for many years.
For this kind of pump, there is a known way of using what is called port plate distribution, the pistons being carried by a barrel which is driven in rotation, the rear face of the barrel being pressed by a spring against a port plate consisting of a disk with curved openings.
The delivery pressure exerts a force, proportional to the delivery pressure and to the surface of the distribution port, on the surface of the barrel facing the distribution port connected to the outlet aperture. This force tends to separate the barrel from the port plate if it exceeds the thrust exerted by the pump pistons.
The delivery pressure also exerts a force on the end walls of the piston housings, this force being proportional to the number of pistons subjected to the delivery pressure. The number of pistons subjected to the delivery pressure varies during the rotation of the barrel.
Consequently, the force exerted on the barrel and tending to press the barrel against the port plate varies during the rotation of the barrel, thus disturbing the operation of the hydraulic pump and possibly accelerating the ageing of the port plate.
The known solution in which the number of pistons is increased to reduce the variation of the force tending to press the barrel against the port plate during the rotation of the barrel is unsatisfactory, since it contributes to an increase in the production cost of the pump.